The three Norns read the thread of destiny and remember how Wotan sacrificed one eye to drink at the Spring of Wisdom and made his spear from a branch of the World Ash Tree. When they remember Alberich’s curse, the thread of destiny snaps and they return to Erda, their mother. At sunrise, Siegfried is about to embark down the Rhine. As a token of her love, Brünnhilde gives him her horse, Grane. Siegfried gives Brünnhilde the ring as a token of his.
Hagen, son of Alberich, urges his half-brother King Gunther and his half-sister Gutrune to marry Brünnhilde and Siegfried, but he does not reveal the existing relationship between the pair. When Siegfried arrives at Gunther’s castle, Gutrune, on Hagen’s advice gives Siegfried a magic drink that erases his memory of Brünnhilde and causes him to fall in love with Gutrune. Siegfried offers to help Gunther win Brünnhilde for his wife if, as a reward, Siegfried may have Gutrune as his own. Gunther and Siegfried drink an oath of blood brotherhood and depart for Brünnhilde’s rock, leaving Hagen to watch over the castle.
Brünnhilde hears the familiar call of Waltraute, one of her Valkyrie sisters, as she approaches the rock. Waltraute tells her of the doom facing Valhalla and the gods. She pleads with Brünnhilde to return the ring to the Rhine Daughters and end the power of Alberich’s curse, but Brünnhilde will not relinquish Siegfried’s pledge of love. Waltraute leaves and Siegfried appears, disguised as Gunther by the Tarnhelm.
When Brünnhilde asks who he is, he says he is Gunther, a Gibichung who will force her to marry him. They struggle, and he rips the ring from her finger. Alone, he calls upon Nothung to bear witness that he will respect his oath of blood brotherhood with Gunther. Then he follows Brünnhilde into her cave.
Alberich appears to Hagen and urges his son to win back the ring from Siegfried. Siegfried returns, eagerly anticipating his wedding to Gutrune. When Siegfried explains that Brünnhilde and Gunther are not far behind, Hagen summons Gunther’s courtiers to pay homage to their new queen. Gunther and Brünnhilde arrives, and she is startled to find Siegfried. Seeing the ring on his finger, she knows she has been betrayed. She declares that Siegfried, not Gunther, won her as his wife and made love to her. Siegfried and Brünnhilde swear on Hagen’s spear that they are speaking the truth. Siegfried leads the guests into the wedding feast, and Brünnhilde, Gunther, and Hagen are left alone. When Hagen offers to avenge Brünnhilde by murdering Siegfried, she confides that her magic has made Siegfried invulnerable except on his back, which she knew he would never turn to any enemy. The three plot Siegfried’s death.
Separated from a hunting party, Siegfried comes upon the Rhine Daughters, who beg him to give back the ring. Although they threaten him with the ring’s curse, he keeps it to prove he’s not afraid. Siegfried tells his fellow hunters about his youth, and Hagen restores his memory with another drink. Siegfried then recalls how he first won Brünnhilde. As two ravens fly out of a thicket, Siegfried turns to watch them, and Hagen plunges his spear into the hero’s unprotected back. The dying Siegfried relives the experience of waking Brünnhilde.
When Siegfried’s body is returned to the hall, Hagen admits killing him and claims the ring as his own, but Gunther opposes him. They fight, and Gunther is killed. Hagen reaches for the ring, but the dead Siegfried’s hand rises in warning and Hagen backs away.
Brünnhilde arrives, now fully aware of everything that has happened. At her command, a funeral pyre is built. She praises Siegfried even as she regrets his treachery. Then she addresses the gods, telling them that she will give them the peace Wotan desires. After removing the ring Siegfried’s finger, she puts it on her own and sets the pyre aflame. She walks into the flame to join Siegfried in death, bequeathing the ring to the Rhine. The Rhine overflows. Hagen attempts to seize the ring, but the Rhine Daughters drag him down and recover the ring. The flames flare up, and Valhalla is destroyed. Brünnhilde’s sacrifice has finally cleansed the ring of its curse and restored the natural order.